* contrib/lisp/org-contacts.el (org-contacts-complete-tags-props): New
function that allows the user to retrieve contacts based on tags and
properties.
(org-contacts-complete-functions): Add
org-contacts-complete-tags-props to the completion functions.
(org-contacts-tags-props-prefix): Prefix used to detect the
completion method wished.
Based on the idea and implementation of John Kitchin